In this project we will study the role of specific brain stem neuronal assemblies, namely the nucleus raphe magnus (NRM) and the region of the nucleus tractus solitarius (NTS) including the area postrema in pain modulation. Single-unit and multi-unit studies will be conducted to ascertain the impact of electrical and chemical stimulation of the AP-NTS upon cellular activity in the NRM. Behavioral studies wll be conducted to quantify the analgesic effect of AP-NTS stimulation, using tail flick and liminal escape response. In addition, power spectral analysis techniques will be used to quantify the changes in EEG synchronization brought about by AP-NTC stimulation. These electrophysiological and behavioral studies will be carried out in acute and chronic rat preparations to determine: (1) whether electrical stimulation of the AP-NTS induces analgesia and whether this effect can be related to modifications in the cellular activity in the more well established NRM-analgesia system, (2) whether chemical stimulation of the AP-NTS can induce analgesia, (3) whether the transmitters involved are of the opiate, serotonergic or noradrenergic class, and (4) whether EEG synchronization is merely a correlate of analgesia or whether hypoarousal is directly contributory to the analgesia mediated from AP-NTS. These combined studies should provide us with a better understanding of the role of the AP upon pain modulation and permit us to develop a more complete model of the elements involved in the descending pain suppressive system.